Add comment to RubyLib.h about #ifdef used.
Add comment to RubyLib.h about #ifdef used.

SketchUp/ruby-c-extension-examples,SketchUp/ruby-c-extension-examples,thomthom/ruby-c-extension-examples,thomthom/ruby-c-extension-examples,thomthom/ruby-c-extension-examples,SketchUp/ruby-c-extension-examples